The present invention relates to a cassette type sheet feed apparatus for a copying machine or the like.
Cassette type sheet feed systems are known in the art which comprise a plurality of cassettes containing different sizes of sheets. Each cassette has a bottom wall formed with a hole and a plate having a front edge aligned with the front of the cassette and a rear edge aligned with the rear of the cassette and tiltably connected to the bottom wall. An actuator is insertable through the hole to tilt the plate and sheets upwardly so that the top sheet engages with the feed roller. Each cassette is designed to contain stacks of sheets of a predetermined width and various lengths. The width of each plate is equal to the width of the corresponding sheets and the length of the plate is equal to the length of the longest sheet of the corresponding width. For sheets of less than maximum length, a rear edge guide is mounted on the plate at an intermediate position to align the front edges of the sheets with the front edge of the plate. The force pressing the sheets against the feed roller is equal to the difference between the upward force applied to the actuator for tilting the plate and the downward force transmitted to the actuator by the plate corresponding to the distributed weight of the sheets on the plate and the resulting moment about the rear edge of the plate. It is clear that wider and longer sheets are heavier and will exert a greater weight on the actuator. In a practical sheet feed apparatus, the upward force is applied to the actuator by a spring and is therefore constant. Therefore, the force pressing the sheets and roller together will be greatest for the narrowest and shortest sheets and smallest for the widest and longest sheets. This condition is extremely disadvantageous from the viewpoint of reliability of sheet feed, and if the range of sizes of the sheets is great enough the smallest sheets will be creased or torn and the largest sheets will not be fed resulting in sheet jams.
Various expedients have been proposed in the prior art to overcome this problem, one being to provide a plurality of actuator biasing springs and a mechanical switching device to provide the proper spring or combination of springs to bias the actuator in accordance with the size of the sheets in the cassette. Another expedient is to provide a single spring and means for varying the preload thereof in accordance with the size of the sheets. Both of these arrangements suffer from the drawbacks of complication and high cost to manufacture. To overcome these drawbacks, compromises are often implemented which result in less than satisfactorily reliable sheet feed.